Frequency modulation system



L. K. SANDOR Filed April 2o,

FREQUENCY MODULATION SYSTEM OOUIIOQ oct. 13, 1942.

l l" INVENToR l g BY Q KTTREY k PSU f wk Patented v Oct. 13, 19422,299,059 FREQUENCY MODULATIQN SYSTEM Louis K. Sandor, Piqua,

Ohio, assignor of seventeen and one-half per cent to Ray C. Stotts andseventeen and one-half per cent -to Harry Friedman, both of Dayton, OhioAppiication Api-i1 zo, 1940, serial No. 330,718

4 5 claims. (01.250-20) This invention relates to a frequency modulationbroadcasting system and more particularly to a radio receiver for usewith frequency modulated broadcasting systems.

In order to produce the wide band width, which virtually is the same asamplitudeamplication, it is necessary to expand or multiply the signal agreat number of times. These signals are received by radio receiversadapted to discriminate the frequency modulated signal so as to producean intermediate or audio frequency signal.

An object of the system disclosed herein is to eliminate some of themultiplication of the signal at the broadcasting station and instead,multiply or expand the band width of the signal at the receiving stationor in the radio receiver.

Another object of this invention is to provide a radio receiver providedwith means for expanding or multiplying the frequency of the incomingsignal.

Another object of this invention is to amplify the incoming signal,followed by one or more stages for expanding or multiplying theamplified signal and then passing the expanded signal through a detectorcircuit.

Another object of this invention is to provide a broadcasting system ofmodulated4 frequencies wherein comparatively narrow band Widths aretransmitted, the band width being broadened or expanded at the radioreceiver.

Other objects and advantages reside in the construction of parts, thecombination thereof and the mode of operation, as will become moreapparent from the following description.

In the drawing, Figure 1 is a schematic showing illustrating theprinciple incorporated into a radio receiver.

Figure 2 is a Wiring diagram of a circuit that has been shown for thepurpose of illustration, carrying out the system shown schematically inFigure 1.

Figure 3 discloses another schematic system wherein the same radioreceiver may be used for receiving amplitude modulated signals whenusing one channel and frequency modulated signals when using the otherchannel.

- All successful systems of frequency modulation used up to the presenttime, so far as known to applicant, have been those which utilize a widecarrier band. The volume or amplitude of the signal is contained in theband width. With a given carrier signal input, the greater the bandwidth, the greater will be the volume output of the detector of thefrequency modulation receiver. Frequency modulation system also has lessnoise from the tubes of the receiver itself, for the reason that thedetector is designed to respond to changes in frequency of constantamplitude and not to changes in amplitude. For example, it hasy beenfound that for satisfactory reception., the band Width should be equalto ve times the highest audio frequency. This width gives a satisfactoryratio of signal to noise level. This Wide band width limits the numberof broadcasting channels that are available.

If it were possible to transmit frequency modulated signals with asmaller band width without sacrificing' the advantages obtained fromfrequency modulated signals of the broad band width now used, thesmaller band Width would be very beneficial to our radio broadcastingsystems. With that thought in mind, experiments have been conducted todetermine the results obtained by using the smaller band width than isnow generally used. It has been found that a frequency modulated signalthat swings over a band width of ten kilocycles. has advantages overamplitude modulated signals, as far as the noise level is concerned.This ten kilocycle band width was received on a conventional frequencymodulated receiver, having the output from the modulated receiveramplified, as in the conventional radio receivers now in use. However,instead of receiving the ten kilocycle band Width frequency modulatedsignals on ordinary frequency modulated receivers now in use, it hasbeen found that very satisfactory results have been obtained by passingthis ten kilocycle band width frequency modulated signal through areceiver provided with several stages of band expanders or frequencymultipliers, such as frequency doublers or frequency quadruplers,connected in series. By this system, the band width is expanded to anydesired width, so as to provide satisfactory amplitude in the form ofband Width for satisfactory detection. For each time' that the frequencymodulated signal is doubled, the band width is doubled, andconsequently, the volumeA of the detector output is automaticallyincreased proportionately.

This system has been exemplified schematically in Figure 1, where theantenna I0 supplies a signal to a receiver circuit I2, which may includea tuning circuit and one or more stages of radio frequencyamplification. It may also include an oscillator detector circuit,having an intermediate frequency output, which may be supplied directlyto band expanders or, frequency multipliers or first amplified and thensupplied to band expanders or frequency multipliers |4. The number offrequency multipliers is purely a matter of choice, depending upon thedesired output. 'Ihe number of stages of band expanding devices I4depends in part upon the width of the received band and the desiredamplitude of the current output.

Band expanding or frequency multiplication corresponds to amplitudeamplification used in conventional radio receivers for receiving anamplitude modulated signal. However, the frequency modulated signal,upon being multiplied, does not suffer from tube distortion and is notaiected by tube noises. Furthermore, extraneous disturbances do not haveas great a detrimental eifect upon frequency multiplication as they doupon amplitude amplification. That being the case, frequencymultiplication has substantially all of the advantages of amplitudeamplification without the disadvantages caused by extraneousdisturbances and defective amplification, tube distortion, tube noisesand the like. The signal output from the frequency multipliers or bandexpanders |4 is supplied to a current limiting device I6 and possiblycurrent filters. This current limiting device is used to energize adetector supplying power to an audio frequency interpreting device 20.

The details of a circuit arrangement shown for the purpose ofillustration have been shown in Figure 2, wherein the receiver circuitincludes one stage of radio frequency amplification, adetectoroscillator stage having an intermediate frequency output and onestage of intermediate frequency amplification. The multipliers or bandexpanding devices include a stage of quadruple multiplication, followedby a doubler. If, for example, the intermediate frequency is selected at240 kilocycles, the resulting output frequency from the doubler stagewould be 1920 kilocycles, resulting in expanding the band eight times.The expanded signal is supplied to a current limiting device thatenergizes the frequency modulated detector circuit having its outputconnected to an audio signal perceptible device, such as a loud speaker.

'Ihe several stages referred to above will now be more fully described.'I'he antenna is connected in series to ground through a primary winding30 coupled to a resonant circuit including a secondary winding or coil32 arranged in parallel with a variable condenser 34 used in tuning thecircuit to the desired incoming signal. One terminal of the resonantcircuit is connected to -the grid of the radio frequency amplifying tube36, which may be of any suitable type, as for eX- ample a GSK? tube. Thecathode is connected through a resistance 38 to the other terminal ofthe coil 32. A suitable by-pass condenser 40 grounds the circuit.

The anode or plate circuit of the tube 36 contains the primary coil 50coupled to a resonant circuit including the secondary winding or coil52, connected in parallel with a variable condenser 54. The output ofthis resonant circuit is connected to the oscillator-detector tube 56,which may be any suitable tube, as for example, a tube known in thetrade as 6A8. The cathode of this tube is connected through a resistance58 to ground. This resistance 58 is shunted bv a bypass condenser 60.'I'he cathode of the tube 56 is also connected through a resistance 62to an electrode of the tube 56 and grounded through a condenser 64connected in series with the reso- 75 nant circuit including theelements E6, 60 and the variable condenser 10. The oscillating anode ofthe tube 56 is coupled to the coil 66 through inductance windings 12,connected in series to ground by a, series of resistances 14, 16 and 13.A suitable. by-pass condenser connects one terminal of the inductance12' to ground.

The output frequency of the tube 56 is an 1ntermediate frequency. Thisintermediate frequencymay be selected at will. For the purpose ofillustration, 240 kilocycles have been used. The anode of the tube 56 isconnected to the resonant circuit including the primary windingconnected in parallel with a condenser 02. The primary winding 90 iscoupled to a resonant circuit including the secondary coil or winding 96connected in parallel with the adjustable condenser 98. The output ofthe resonant circuit including the coil 96 and the condenser 08 isconnected to the grid of an amplifying tube |00, which may be a GSK?tube, for amplifying the intermediate frequency. The cathode .of thetube |00 is conected to. ground through a suitable resistance |02connected in parallel with the bypass condenser |04.

The output circuit of the tube |00 is connected in series with anadjustable condenser ||0 connected in parallel with the prmary windingor coil ||2, coupled to a resonaml circuit including the secondarywinding or coil ||4 and the adjustable condenser H8. A suitableresistance ||6 is connected in parallel with the variable condenser l0and the primary windingl 2, so as to broaden the band width of theresonant circuit, as is well known to those skilled in the art. 'I'heoutput of the resonant circuit including the coil |4 andthe condenser||8 is connected to a grid of the tube |20 having an output frequencyfour times the frequency of the original intermediate frequency.Assuming the intermediate frequency is 240 kilocycles, the output of thetube |20 will then have a frequency of 960 kilocycles. anode circuit isconnected to a resonant circuit including the adjustable condenser |22and the coil |24. 'I'he resonant circuit is connected between the anodeand the screen grid of the tube I 20. Furthermore, the output circuit ofthis tube |20 is connected through a condenser |26 to the controlelectrode of a tube |28, which may be of the 6SJ7 type. The controlelectrode is grounded through a suitable grid leak |30.

The output of the tube |23 has a frequency twice that of the input, thatis, 1920 kilocycles. 'Ihis output is connected through a pair ofcondensers |32 and |34 to ground. An inductance |36 is connected inparallel with the adjustable condenser, |32, so as to form a resonantcircuit, connected by a suitable condenser |40 to the current limitingtube |42. This tube may also be of the 6SJ7 type. A suitable resistance|44 is also connected in parallel with the adjustable condenser |32 andthe inductance |36, so as to broaden the band width of this circuit. Thecathode of the tube |42 is grounded and the control electrode isconnected to ground through a suitable grid leak resistance |46. Theoutput of the tube |42 is connected to a resonant circuit including theadjustable condenser |48 and the primary winding or coil |50.

. Theoutput of the current limiting tube |42 is supplied to the detectorby the coil |50 being inductively coupled to the secondary windings |52and |54. 'I'he terminal of one Winding is connected to one anode of thedetector tube |60 and the terminal of the other winding is connected toThe' the other anode of the detector tube |60. This detector tube may beof the type known as 6I-I6. The center tap of the secondary windings |52and |54 is connected through a suitable inductance |62' to the circuitsof the cathodes of the tube |60. One of these circuits includes theresistance |10 shunted by a by-pass condenser |12. The cathode in thiscircuit is grounded. The other of these circuits includes the resistance|14 shunted by a by-pass condenser |16. The cathode in this othercircuit is grounded through the resistance |64 and variable resistance|66 connected in series with a condenser |68. The resistance |66 isprovided with an adjustable tap for supplying audio signal current' toany suitable audio perceptible signal device. The inductance |62 isconnected by a condenser |80 back to the anode of the tube |42.

Frequency expanding device or frequency expanding stages have been usedto describe a system for expanding the band width in a receiver, whichin some respects isthe equivalent of frequency multipliers intransmitting devices. However, the final signal supplied to the detectorneed not necessarily be higher than the signal input of the receiver.ultimate signal supplied to the detector may not be as high, and if so,is not a multiple of the signal input into the receiver. Therefore, bandwidth expanding device, or frequency expanding device, has been usedherein to designate any suitable device for widening the band width,whether or not the ultimate signal supplied to the detector is amultiple of, lower than or higher than the input signal that is pickedup at the antenna.

In the modification disclosed in Figure 3, a dual purpose radio receiverhas been shown. When the input signal is a frequency modulated signal,it passes through the upper branch of the circuit through the frequencyexpanding devices, through the detector to the output. If the incomingsignal is an amplitude modulated signal, it is passed through the lowerbranch -of the receiver, which may be a conventional 'superheterodyne ora tunedradio frequency receiver, or any other suitable receiver forreceiving amplitude modulated signal, amplifying the same and convertingthis signal either directly or indirectly into an audio perceptiblesignal through any suitable medium. The upper branch may consist of acircuit similar to that described in connection with Figure 2, or anyother circuit for expanding the band widths of the frequency modulatedincoming signal. may function as the receiver for frequency modulatedincoming signals converted into audio perceptible signals through theband'expandin'g device, or for converting the incoming amplitudemodulated signal into an audio perceptible signal through the use of aconventional amplitude modulated signal receiving circuit.

Although the preferred modification of the device has been described, itwill be understood that within the purview of this invention variouschanges may be made in the form, details, proportion and arrangement ofparts, the combination thereof and mode of operation, which generallystated consist in a device capable of carrying out the objects setforth, as disclosed and defined in the appended claims.

Having thus described my invention, I claim:

1. A super-heterodyne radio receiver including means for selecting aradio frequency signal,

Thus, the same receiver That being the case, the

means including an oscillator-detector circuit for converting theselected signal into an intermediate frequency signal, a selectedcircuit selected from a pair of parallel circuits, one for expanding theband width of a frequency modulated signal and the other for amplifyingan amplitude modulated signal, and means for converting the signaloutput of the selected circuit into perceptible signals.

2. In a radio receiver for use either with a frequency modulated signalor an amplitude modulated signal, said receiver having a pair ofpa'rallel circuits, one for use with a frequency modulated signal andthe other for use with the amplitude modulated'signal, said radioreceiver including a circuit network including a by-pass band filter forselecting a broadcast signal, means including an oscillator-detectorcircuit for converting the selected signal into an intermediatefrequency signal, a circuit selected from a pair of circuits, one forexpanding the band width of the frequency modulated signal, the otherfor amplifying an amplitude modulated signal, and a detector circuit forconverting the current output of the selected circuit into perceptiblesignals.

3. A super-heterodyne radio` receiver including means for selecting aradio frequency signal, which signal may be either a frequency modulatedsignal or an amplitude modulated signal, means including anoscillator-detector circuit for converting the selected signal into anintermediate frequency signal, a selected circuit selected from a pairof parallel circuits, one of which circuits includes means for expandingthe band width of the selected signal, and current limiting circuitmeans having a substantially constant amplitude current output and theother circuit. for amplifying an amplitude modulated signal, and means:Ofor converting the signal output of the selecteu'circuit intoperceptible signals.

4. A super-heterodyne radio receiver including means for selecting aradio frequency signal, which signal may be either a frequency modulatedsignal or an amplitude modulated signal, said means including a resonantcircuit for selecting the signal, means including an oscillatordetectorcircuit for converting the selected signal into an intermediatefrequency signal, a selected circuit selected from a pair of parallelcircuits, one of which circuits includes means for expanding the bandwidth of the intermediate frequency signal, current limiting meanshaving a substantially constant amplitude signal current output, and adetector for translating the intermediate frequency expanded band Widthinto perceptible signals. y

5. In a radio receiver for use either with a frequency modulated signalor an amplitude modulated signal, said receiver having a pair ofparallel circuits, one for use with .a frequency modulated signal andthe other f'or use with the amplitude modulated signal, said radioreceiver including a circuit network including a by-pass band filter forselecting a broadcast signal, means including an oscillator-detectorcircuit for converting the selected signal into an intermediatefrequency signal, a circuit selected from a pair of circuits, one forexpanding the band width of the frequency modulated signal, said oneincluding current limiting means having a substantially constantamplitude signal current output, the other for amplifying an amplitudemodulated signal, and a detector circuit for converting the currentoutput of the selected circuit into perceptible signals.

LOUIS K. SANDOR.

